This invention relates to a method of desensitizing a pressure sensitive recording sheet and more particularly, to spot-desensitizing of a sheet sensitized with organic or inorganic color forming reactant acid materials which act as electron acceptor in an electron-donor-acceptor color-forming reaction. The invention also relates to desensitized sheets obtained by such desensitization.
There are known several types of pressure-sensitive recording system utilizing an electron donor-acceptor color-forming reaction between a colorless chromogenic material and a color forming reactant acid material. The colorless chromogenic material and the color forming reactant acid material are hereinafter referred to as "color former" and "color acceptor", respectively. Those pressure-sensitive recording systems comprise a support sheet having, disposed on at least one surface thereof, micro capsules containing oil droplets in which a color former is dissolved and/or a color acceptor capable of producing color when brought into reactive contact with the color former.
For example, U.S. Pat. No. 2,730,456 discloses a transfer-copy system wherein minute oil droplets in which a color former is dispersed or dissolved are encapsulated and coated onto a transfer sheet. The color former is thereafter transferred to a receiving sheet by rupturing said capsules. The underlying receiving sheet has a color reactant coating thereon containing a color acceptor which will react with the color former causing a visible colored mark at points where the microcapsules have been ruptured and the color former has been transferred. In case when multiple copies are needed at least one intermediate copy sheet having said microcapsules and said color acceptor on the opposite surfaces thereof, respectively, is interposed between the upper copy sheet and the underlying copy sheet.
U.S. Pat. No. 2,730,457 discloses another type of a pressure-sensitive record sheet. In this pressure-sensitive record sheet, there are disposed on one surface of the same sheet both the color acceptor and the capsules containing oil droplets in which the color former is dissolved. This record material is known as a "self contained" system.
It is known to locally apply a desensitizer in advance onto the color-acceptor-coated surface in order to inactivate color-forming reactivity of the color former on the area where color-forming reaction is not required. The desensitizer may also be applied locally onto the color-acceptor-coated surface where a color image has been produced in order to erase the image.
Desensitizers which have been conventionally used are polyethylene glycol, glycerol, dodecyl trimethylammonium chloride, dodecyl amine, dodecyl dimethylamine, N,N-bispolyoxyethylene alkylamine, N,N,N-trispolyoxyethyleneamine, N,N,N'-trispolyoxyethylene N-alkylenediamine, N,N,N',N',-tetrakispolyoxyethylene alkylenediamine, N-alkyl-N,N',N',-trimethylalkylenediamine, etc. Among those desensitizers, ethylene oxide adducts of amine, diamine and polyamine are particularly superior in the desensitizing effect, but those adducts are not suitable for offset printing because hydrophobic vehicle can not be used in making ink due to many hydrophilic groups of those adducts. In case that the pressure-sensitive recording sheet which is desensitized with the above mentioned adducts is placed in the condition of a high temperature and a high humidity, hygroscopic phenomenun by hydrophilic desensitizer becomes more remarkable and therefore the desensitizer applied onto the area where color-forming reaction is required migrates so far to the adjacent areas where color-forming reaction is not required, with the result that the ability of color-forming reaction at such the adjacent areas is lowered. In addition, as capsules on the back surface or the adjacent sheet are left in the extroadinary high humidity because of the hygroscopic property thereof, the spontaneous rupture of capsules will occur easily.
Recently, organic compounds such as aromatic carboxylic acids or polymers of organic acids such as phenol-formaldehyde resin which are more hydrophobic than inorganic solid acids have been used as color acceptor. For example, DT-OS 2147585, 2152765 and 2242250 disclose that the mixture of aromatic carboxylic acid derivatives such as benzoic acid, salicylic acid or derivatives of which with metal compounds such as metal oxide, metal hydroxide and metal carbonate; or metallic salts of above aromatic carboxylic acid derivatives can be used as color acceptor. It is also suggested in DT-OS 2148427 to use the mixture or copolymer of aromatic carboxylic acid, and aldehyde or acetylene with metallic compounds, and in DT-PS 2152763 to use metallic salts of the said polymer.
The above-mentioned conventional desensitizers are disadvantageous in that they remain sticky for a long time because of the poor affinity for the said organic acids and polymers thereof. Long chain alkyl-dimethyl amine and N-long chain alkyl-N,N',N'-trimethylalkylenediamine are rather hydrophobic but they have an undesirable smell inherent to amines. Especially, long chain alkyl dimethylamine does not show a satisfactory disensitizing effect.
The primary object of the invention is to provide a new and improved method for desensitizing a pressure sensitive recording sheet in which the above mentioned disadvantages with conventional desensitizers can be avoided and an excellent desensitizing effect is obtained.
Another object of the invention is to provide a new desensitizer which is hydrophobic and well soluble in oil vehicle to make an ink for offset printing use.
A further object of the invention is to provide an improved desensitizer which has a good affinity to the color acceptor coating layer which is made of an organic acid or polymer thereof.
A still further object of the invention is to provide an improved desensitizer ink which is superior in both inksetting and migration-resistance.
One of other objects of invention is to provide an improved desensitizer for the pressure sensitive recording sheet which is odorless.
It is also one of the objects of the invention to provide a partly desensitized, pressure sensitive recording sheet in which migration of the desensitizer applied to the sheet hardly occurs even in condition of a high temperature and a high humidity so that capsules containing oil droplets may be prevented from spontaneous rupture at the area adjacent to the area to which the desensitizer is applied.